Fin field effect transistor (FinFET)

ABSTRACT

A FinFET whose fin has an upper portion doped with a first conductivity type and a lower portion doped with a second conductivity type, and the junction between the upper portion and the lower portion acts as a diode. The FinFET further includes: at least one layer of high-k dielectric material (for example Si 3 N 4 ) adjacent at least one side of the fin for redistributing a potential drop more evenly over the diode. Examples of the k value for the high-k dielectric material are k≧5, k≧7.5, and k≧20.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of U.S. patentapplication Ser. No. 13/063,504, filed on Mar. 17, 2011, which is a PCTNo. PCT/IB2009/053963 (filed on Sep. 10, 2009) entry in US nationalstage, the entire of which are incorporated by reference herein.

The present invention relates to Fin Field Effect transistors (FinFETs)and the fabrication thereof. The present invention is particularlysuited to FinFETs fabricated on bulk silicon wafers typically used forfabricating planar bulk FETs.

A FinFET is a field effect transistor which has a narrow, active area ofa semiconductor material protruding from a substrate so as to resemble afin. The fin includes source and drain regions. Active areas of the finare separated by shallow trench isolation (STI), typically SiO₂. TheFinFET also includes a gate region located between the source and thedrain regions. The gate region is formed on a top surface and sidewallsof the fin such that it wraps around the fin. The portion of the finextending under the gate between the source region and the drain regionis the channel region.

FinFETs are regarded as main candidates to replace conventional planarbulk MOSFETs in advanced (beyond 32 nm node) CMOS thanks to their goodgate control over the channel, resulting in improved short-channeleffect immunity and I_(on)/I_(off) ratio.

One type of FinFET is fabricated on silicon on insulator (SOI) wafers.One advantage of SOI FinFETs is that they have low leakage current fromsource to drain because there is an oxide layer below the fin whichblocks the leakage current.

Another type of FinFET is fabricated on conventional bulk siliconwafers. These FinFETs are known as bulk FinFETs. Fabricating FinFETs onconventional bulk Si wafers can be considered advantageous for tworeasons: (i) the lower cost of bulk wafers and (ii) the option toco-integrate conventional planar bulk FETs and FinFETs in a singleproduct.

In FinFETs, the source and the drain region are heavily doped. Thesource and the drain regions each have a first conductivity type (n-typefor NMOS and p-type for PMOS). A problem with existing bulk FinFETs isthat a leakage path from source to drain exists through the part of thefin which is not controlled by the gate, i.e. the portion of the finbelow the gate and adjacent to the STI. The leakage from source to drainthrough the lower part of the fin is known as punch-through leakage.Punch-through leakage causes an increase of static power consumptionwhich is undesirable.

In order to solve the problem of punch-through leakage in bulk FinFETs,a lower portion of the fin is doped to have a conductivity type oppositethe conductivity type of the source and drain regions (p-type for NMOS;n-type for PMOS). The punch-though-stopper (PTS) dopant is implanted inthe part of the fin directly below the channel and below the source anddrain regions.

A result of PTS doping, however, is that, in both the source region andthe drain region of the fin, there is an abrupt junction between theupper part of the fin of the first conductivity type and the lower partof the fin of the second and opposite conductivity type. The junctioneffectively operates as a diode located between: the source region andthe substrate; and the drain region and the substrate, respectively.

In digital circuits NMOS substrates are biased to zero volts, and thesource and drain potentials vary between zero and the supply voltageV_(dd). The same holds for PMOS, in which the situation iscomplementary. Thus the diodes are either unbiased or reverse-biased inCMOS circuit application.

When the N++/P+ (NMOS) or P++/N+ (PMOS) diodes are reverse biased thereis a high electric field across the very abrupt n/p junction. As aresult of the high electric field, the conduction and valence bands ofthe n-type and p-type semiconductors are sharply bent and electrons cantunnel from valence band (leaving a hole behind) to conduction band orvice versa. This tunneling can be pure quantum mechanical, or thermallyassisted, or trap-assisted. In the latter case the leakage is enhancedby structural damage in the diode, often generated by the implantationprocess. The effect is known as known as band-to-band tunneling (BTBT).Band to-band tunneling results in a leakage current across thereverse-biased N++/P+ (NMOS) or P++/N+ (PMOS) diodes rendering bulkFinFETs unattractive for low standby power application.

To optimise the leakage of a bulk FinFET, the optimum PTS doping levelis determined by a trade-off between punch-through from source to drain(which demands high doping) and band-to-band tunneling from source/drainto substrate (which demands low doping).

US2006/0118876 A1 discloses a bulk FinFET in which a layers of siliconnitride and oxides are positioned adjacent the source and drain regionsof the fin.

US 2008/0048262 A1 discloses a FinFET in which the source/drain portionsof a fin are coated with an etch stop layer of silicon nitride.

The present inventors have realised it would be desirable to reduce theleakage due to band-to-band tunneling.

The present inventors have also realised it would be desirable toimprove the trade-off between punch-through (which demands high doping)and band-to-band tunneling (which demands low doping).

In a first aspect, the present invention provides a FinFET comprising: asemiconductor substrate with a fin; the fin having an upper portion anda lower portion, the upper portion being doped with a dopant of a firstconductivity type, the lower portion being doped with a dopant of asecond conductivity type, wherein the junction between the upper portionand the lower portion acts as a diode. The FinFET further comprises: atleast one layer of high-k dielectric material adjacent to at least oneside of the fin for redistributing a potential drop more evenly over thediode, compared to if the at least one layer of high-k dielectricmaterial were not present, when the upper portion is connected to afirst potential and the lower portion is connected to a second potentialthereby providing the potential drop across the junction.

The at least one layer of high-k dielectric material may have a k valueof k≧5.

The at least one layer of high-k dielectric material may have a k valueof k≧7.5.

The at least one layer of high-k dielectric material may have a k valueof k≧20.

The at least one layer (26, 28) of high-k dielectric material may beHfO₂.

The at least one layer of high-k dielectric material adjacent at leastone side of the fin may comprise a layer of dielectric material providedadjacent opposite sides of the fin.

The FinFET may further comprise a shallow trench isolation layerprovided above the substrate and adjacent the layer of high-k dielectricmaterial.

The fin may further comprise a source and a drain separated by a channelregion, the channel region of the fin being surrounded by a gate regionon three sides.

The FinFET may further comprise a punch through stopper layer providedin the lower portion of the fin below the channel region.

In a further aspect the present invention comprises a method offabricating a FinFET, the method including the steps of: providing asemiconductor substrate; etching the substrate to provide a fin;depositing a layer of high-k dielectric material adjacent at least oneside of the fin; depositing a shallow trench isolation layer above thesubstrate and adjacent the layer of high-k dielectric material;providing a gate region on top of and around the sides of the fin; andimplanting dopants in the fin to form the active semiconductor areas.

The step of implanting dopants may comprise heavily doping an upperportion and a lower portion of the fin, the upper portion being dopedwith a dopant of a first conductivity type, the lower portion beingdoped with a dopant of a second conductivity type, wherein the junctionbetween the upper portion and the lower portion acts as a diode; andwherein the step of depositing a layer of high-k material comprisesdepositing at least one layer of high-k dielectric material adjacent atleast one side of the fin for redistributing a potential drop moreevenly over the diode, compared to if the at least one layer of high-kdielectric material were not present, when the upper portion isconnected to a first potential and the lower portion is connected to asecond potential thereby providing the potential drop across thejunction.

The step of depositing a layer of high-k dielectric material maycomprise depositing a layer of high-k dielectric material with a k valueof k≧5.

The step of depositing a layer of high-k dielectric material maycomprise depositing a layer of high-k dielectric material with a k valueof k≧7.5.

The step of depositing a layer of high-k dielectric material maycomprise depositing a layer of high-k dielectric material with a k valueof k≧20.

The step of depositing a layer of high-k dielectric material adjacent atleast one side of the fin may comprise depositing a layer of high-kdielectric material adjacent opposite sides of the fin.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view (not to scale) of a first embodiment of abulk FinFET;

FIG. 2 is a schematic illustration (not to scale) of a cross-sectiontaken through the fin outside the gate of the bulk NMOS FinFET of FIG.1;

FIG. 3 is a schematic illustration of a method of fabrication of theFinFET illustrated in FIGS. 1 and 2;

FIG. 4 is a schematic illustration (not to scale) of a cross-sectiontaken through the fin outside the gate of a second embodiment of aFinFET;

FIG. 5 is a schematic diagram of the simulated reverse bias diodecharacteristics for various FinFETs;

FIG. 6 is a schematic diagram demonstrating the RESURF effect around n/pdiode junction for two FinFETs;

FIG. 7 is a schematic illustration of a 2-input NAND logic gate in CMOStechnology which is illustrative of the possible advantages of theleakage reduction demonstrated in FIG. 5 on standard cell level; and

FIG. 8 is a full three-dimensional simulation of the total leakagecurrent of the top NMOS transistor in FIG. 7, where a fin width of 15nm, a typical value for the 22 nm node has been chosen.

FIG. 1 is a schematic view (not to scale) of a first embodiment of abulk FinFET 100. The FinFET 100 comprises a substrate 20 of silicon. TheFinFET 100 includes a narrow rectangular fin 12. Fins are typicallybetween 10 nm and 30 nm wide. In this embodiment the fin 12 isapproximately 20 nm wide. The fin 12 protrudes from the substrate 20 ina direction perpendicular to the plane of the substrate 20. The activetop part of the fin 12 is 50 nm high (typically for fins this is 40-60nm). The bottom part of the fin 12 is approximately 200 nm high. The fin12 includes a source region 14 and a drain region 16. The source anddrain regions 14, 16 of the fin 12 are separated by a layer 18 of SiO₂,known as the shallow trench isolation or the STI 18. In this embodimentthe STI 18 has a thickness of approximately 250 nm. Between the STI 18and the fin there is a 10 nm wide layer 26 of HfO₂ (a high-k dielectricmaterial) which extends along the length of the lower portion of the fin12. The HfO₂ has a typical k value of k=21. The high-k dielectric layer26 also extends between the substrate 20 and the STI 18. The high-kdielectric layer 26, which is not present in known FinFETs, will bedescribed in more detail with reference to FIG. 2.

A gate region 22 is located between the source region 14 and the drainregion 16. The gate region 22 is located on top of the STI 18. The gateregion 22 extends over and across the fin 12. The fin 12 also comprisesa channel region 24 located between the source region 14 and the drainregion 16 and under the gate region 22. As is shown in FIG. 1 the gateregion 22 wraps around the fin 12 from three sides. As a result the gate22 has excellent electrostatic control over the channel region 24 of thefin 12.

The FinFET 100 illustrated in FIGS. 1 and 2 is an NMOS FET. The sourceregion 14 and the drain region 16 of the fin 12 are heavily doped withn-type semiconductor dopants. The channel region 24 is located betweenthe source 14 and the drain 16 regions of the fin 12 in the upperportion of the fin 12. The channel region 24 can be undoped or lightlydoped.

As explained above, a problem with known bulk FinFETs is that at highbias voltages FinFETs suffer from punch-through leakage. In known bulkFinFETs, a leakage path exists from the source region to the drainregion through the part of the fin which is not controlled by the gate,i.e. the portion of the fin adjacent to the STI and below the channelregion.

FIG. 2 is a schematic illustration (not to scale) of a cross-sectiontaken through the fin outside the gate i.e. through the source 14 ordrain 16 of the bulk NMOS FinFET 100 of FIG. 1.

In order to eliminate the leakage current occurring in known bulkFinFETs caused by punch through between the source and the drainregions, the fin 12 of the FinFET 100 is subdivided into two portions30, 32.

The upper portion 30 of the source and the drain regions 14, 16 ishighly doped by an ion implantation process with n-type dopant. A p-typepunch-through stopper (PTS) dopant is implanted in the lower portion 32of the fin 12 directly below the channel region 24.

The subdivision of the fin 12 into oppositely doped upper and lowerportions 30, 32, however, causes an abrupt n/p junction 34 to be formedbetween each of the source 14 and drain regions 16 and the lower portion32 of the fin. As is shown schematically in FIG. 2, the abrupt n/pjunction 34 is located slightly below the surface level of the STI 18.

The purpose of oppositely doping the upper and lower portions 30, 32 ofthe fin 12 is that the junction 34 between the differently dopedportions of the fin 12 inhibits leakage current between the source 14and drain 16.

A disadvantageous effect of the highly abrupt n/p junction 34, however,is that the junction 34 acts as a diode, as shown schematically in FIG.2.

As explained above, the abruptness of the junctions formed between thesource and drain regions 14, 16 and the lower portion 32 of the fin 12leads to leakage currents formed by band-to-band tunneling (BTBT).Although BTBT could be reduced by lowering the doping levels in thesource and drain regions 14, 16, this step would also lead to seriesresistance between source/drain and channel, while reducing thepunch-through stopper dose would reduce the efficacy of the punchthrough stopper (PTS) layer protection described above.

In standby, the largest potential drop over the reverse-biasedsource/substrate and drain/substrate diodes equals the supply voltageV_(dd). However, band-to-band tunneling is driven essentially by theelectric field, which is the spatial gradient of the potential. Thatmeans that if the potential drop can be redistributed more evenly overthe diode the band-to-band tunneling can be reduced.

In the embodiment of FIGS. 1 and 2, redistributing the potential drop isimplemented by placing a layer 26 of HfO₂ (a high-k dielectric) adjacentto the n/p diode 34.

As can be see in FIGS. 1 and 2, the layer 26 of high-k dielectric isadjacent to and extends along the lower portion 32 of the fin 12. Thehigh-k dielectric layer 26 extends in a plane parallel to the fin 12.The high-k dielectric layer 26 surrounds the junction 34 between theupper portion 30 and the lower portion 32 of the fin 12. The high-kdielectric layer 26 also lies beneath the SiO₂ of the STI trench 18surrounding the lower portion 32 of the fin 12. The high-k dielectriclayer 26 also extends in a plane parallel to the plane of the substrate20. The high-k dielectric layer 26 forms a liner between the STI 18, thefin 12 and the substrate 20.

A high-k dielectric is an insulating material with a high dielectricconstant in comparison to SiO₂ (k=3.9). Examples are silicon nitride(k=7.5) or HfO₂ (k>20). The effect of its high dielectric permeabilityis to force the electric field to penetrate through the high-kdielectric layer 26. As is explained in more detail later with referenceto FIG. 6, the high-k dielectric layer 26 reduces the local electricfield strength near the junction 34. This effect is known as the RESURFeffect (reduced surface field). The RESURF effect lowers the localelectric field strength across the junction 34, thereby reducing theeffects of BTBT without requiring a reduction in doping levels.

FIG. 3 is a schematic illustration of a method of fabrication of theFinFET illustrated in FIGS. 1 and 2.

In step s2, the substrate 20 is provided. The substrate is a bulk Siwafer.

In step s4, the substrate 20 is etched to provide the fin portion 12 ofthe FinFET 100. The fin 12 is a rectangular shaped protrusion ofsemiconductor material extending perpendicularly to the plane of thesubstrate 20.

In step s6, the high-k dielectric layer 26 is deposited on top of thesubstrate 20 and on both sides of the fin 12 along the length of the fin12 so that the high-k dielectric layer 26 is provided directly adjacentthe fin 12 on opposite sides of the upper portion 30 of the fin 12. Thehigh-k dielectric layer 26 is 10 nm thick. Particularly effectivethicknesses for the high-k layer are thicknesses falling in the range of5-10 nm on either side of the junction.

In step s8, the SiO₂ shallow trench isolation (STI) layer 18 isprovided. The STI 18 is formed on top of the high-k layer 26 bydepositing SiO₂ on areas either side of the fin 12 in a similarfabrication method as is used in conventional planar bulk CMOSfabrication. Once the STI 18 has been deposited on top of the high-klayer 26, it is planarised. Unlike in the planar bulk CMOS fabricationprocess, however, in this FinFET fabrication process, the STI is thenetched back after planarisation so that the side walls of the fin 12 areexposed. The high-k dielectric layer 26 HfO₂ acts as a liner positionedbetween the STI 18 and the opposite sides of the upper portion 30 of thefin 12.

In step s10, the gate 22 is provided. Layers of dielectric, metal andpolysilicon material are deposited on top of the STI 18 and across thetop of and around the sides of the fin 12 and etched to form the gate 22illustrated in FIG. 1.

In step s12, after gate etch, dopants are implanted in the fin 12 toform the active semiconductor areas. The source and drain regions 14, 16are heavily doped by an ion implantation process using n-type dopantsfor NMOS and p-type for PMOS.

For the sake of simplicity, the step s12 of doping by ion implantationis explained as occurring after the step s10 of gate etch, since this iswhen the final heavy doping of the source and drain regions 14, 16 ofthe exposed upper portion 30 of the fin 12 typically takes place. Aswill now be explained, however, doping the different types of layers andregions may take place at various stages during fabrication.

For example, the source and drain are implanted after the gate has beenformed, the gate acting as a mask for the channel region which shouldnot receive the source/drain implant. The PTS is implanted before gateformation. The source/drain are implanted more shallowly, but withhigher concentration.

The channel can be implanted before or after gate etch; in the lattercase the channel will not be homogeneously doped (this is known aspockets or halos).

Typically the PTS profile starts directly below the active (top part) ofthe fin and extends between 40 and 100 nm downwards.

Various other implementation details are possible, as follows.

-   -   A WELL may be used. The well is meant to isolate transistors        from each other and to isolate the source/drain from the        substrate. An NMOS is made in a P-WELL, a PMOS is made in an        N-WELL.    -   A VT-Adjust (VTA) may be used, and is intended to tune the        threshold voltage.    -   The PTS is intended to prevent deep leakage between the source        and the drain.    -   The HALO (=POCKET) is intended to make the VT less dependent on        the gate length.

There are many different scenarios in which implantations are done atdifferent steps in the process. The halo is always implanted after thegate is formed, the other three implementations discussed directly aboveare typically (but not necessarily) prior to that.

In a bulk FinFET normally only the WELL and the PTS are implanted, priorto gate deposition. The WELL has a low concentration (typically<1e17/cm³) which is not enough to stop punch-through, the PTS hasconcentrations around 1e18/cm³.

Various options are possible with regard to the timing and theimplementation of the earlier mentioned planarising actions, includingthe following three possibilities:

(i) the high-k layer is deposited to the level of the top of the fin andthen subject to planarisation before the STI layer is applied;

(ii) the STI layer and the high-k layer are etched back simultaneouslyto expose the upper portion of the fin; and

(iii) the high-k layer is deposited only to the level that the STI layerwill be at after etching, the STI then being deposited over the high-klayer, planarised and then etched back to the same height as the high-klayer.

Of the above, option (ii) is the easiest from a manufacturing point ofview.

Returning to consideration of the total process, one particularlysuitable example of the order of various doping and etching steps is thefollowing sequence:

-   -   WELL    -   Fin etch    -   high-k and STI fill and etch back    -   PTS    -   Gate    -   source/drain.

FIG. 4 is a schematic illustration (not to scale) of a cross-sectiontaken through the fin outside the gate i.e. through the source 14 ordrain 16 of a second embodiment of a FinFET 200. Where features are thesame as for the FinFET 100 of FIGS. 1 and 2, the same reference numeralshave been used.

The fin 12 of the FinFET 200 of this embodiment is subdivided into anupper portion 30 and a lower portion 32. The upper portion 30 of the fin12 forms the source and the drain regions 14, 16. The source and thedrain regions 14, 16 are highly doped with n-type dopant (NMOS). Thelower portion 32 of the fin is doped with a p-type punch-through stopperdopant implanted in the lower portion 32 of the fin 12 directly belowthe channel 24.

To prevent the abruptness of the junctions formed between the source anddrain regions and the lower portion of the fin from leading to leakagecurrents formed by band-to-band tunneling (BTBT), a layer 28 of HfO₂(i.e. a high-k dielectric layer) is placed adjacent to the n/p junction34.

As can be see in FIG. 4, the high-k dielectric layer 28 of is adjacentto and extends along the lower portion 32 of the fin 12. The high-kdielectric layer 28 extends in a plane parallel to the fin 12. Thehigh-k dielectric layer 28 surrounds the junction 34 between the upperportion 30 and the lower portion 32 of the fin 12.

In contrast to the first embodiment, in this embodiment, the high-kdielectric layer 28 does not lie beneath the SiO₂ of the STI trench 18surrounding the lower portion 32 of the fin 12. The high-k dielectriclayer 28 does not extend in a plane parallel to the plane of thesubstrate 20. The high-k dielectric layer 28 does not extend to the topsurface of the substrate 20. Thus in this embodiment, the high-kdielectric layer 28 surrounds the lower portion 32 of the fin 12 in theregion of the n/p junction 34 only.

FIG. 5 is a schematic diagram of the simulated reverse bias diodecharacteristics for various FinFETs. The vertical axis 40 shows theleakage current I_(leak) in arbitrary units.

The horizontal axis 42 shows the supply voltage V_(dd), in Volts. Curve46 shows the simulated diode characteristic for FinFET 200 shown incross-section in FIG. 4, in which the high-k dielectric layer is Si₃N₄(k=7.5). Curve 48 shows the simulated diode characteristic for a FinFETconstructed in the same way as the FinFET 200 shown in cross-section inFIG. 4, but in which the dielectric is a material with a very high-kvalue (k=20). Curve 44 shows the simulated diode characteristic for areference FinFET without a high-k dielectric layer.

Clearly, for supply voltages around 1V, for the 22 nm node, diodeleakage can be reduced by approximately 1.5 orders of magnitude, i.e. byapproximately a factor of 30, thanks to the RESURF effect caused by thehigh-k layer.

FIG. 6 is a schematic diagram demonstrating the RESURF effect around n/pdiode junction for two FinFETs. For each FinFET only half of thesymmetric structure is shown. The junction position is indicated by thediode symbol. The vertical axis 52 shows the vertical height Y of theFinFET in micrometers (μm). The horizontal axis 54 shows the horizontaldistance X from the central axis of the fin 12 in micrometers (μm).

The left hand side of the diagram 50 depicts a standard FinFET with SiO₂everywhere in the STI area, i.e. with no high-k dielectric layer. Theright hand side of the diagram 60 shows a cross section of a FinFET witha high-k dielectric layer.

In both the left and the right hand side diagrams, the central portion56 of the FinFETs is a silicon fin. In the left hand side diagram 50,the portions 58 of the FinFET adjacent the central portion 56 are SiO₂.In the right hand side diagram 60, the portion 62 of the FinFET adjacentthe fin portion 56 is a 10 nm wide high-k dielectric layer or liner. Thehigh-k insulating liner is adjacent to the p/n junction. The portion 58of the FinFET adjacent the high-k dielectric layer 62 but further fromthe fin portion 56 is SiO₂.

The diagram shows the distribution of the iso-potential lines acrosseach FinFET. It is clear from the diagrams that the iso-potential linesfor the FinFET with high-k liner, as shown in the right hand FIG. 60,are distributed over a larger distance in the y-direction, demonstratingthe RESURF effect.

Simulations show that a leakage improvement of between 10× to 100× canbe achieved for all combinations of fin width between 10-30 nm,punch-through stopper (PTS) concentrations between 10¹⁸ and 10¹⁹atom/cm³ and source/drain doping levels around 10²⁰ atom/cm³. Also themechanism of tunneling (whether direct tunneling or trap-assistedtunneling) plays no role in the obtainable improvement.

FIG. 7 is a schematic illustration of a 2-input NAND logic gate 70 inCMOS technology which is illustrative of the possible advantages of theleakage reduction demonstrated in FIG. 5 on standard cell level.

In FIG. 7 two PMOS transistors 72 are connected in parallel, with theirsubstrate contacts at the logical “1”. Two NMOS transistors 74, 76 areconnected in series with their substrate contacts at the logical “0”. Inthe depicted state of the cell, three source/drain to substrate diodesare reverse-biased and leak to the substrate contact, as indicated byarrows 78. Thus, in the bias condition depicted in the 2-input NANDlogic gate 70 shown in FIG. 7, three out of four NMOS source/drain tosubstrate diodes are reverse-biased and will contribute to the totalleakage of the cell.

Note that the situation depicted in FIG. 7 is only illustrative. Forother bias conditions other diodes will leak, and for any digital cellin CMOS technology, whether SRAM or logical, there are source/draindiodes which are reverse-biased, their leaking current contributing toand possibly dominating the total standby power consumption.

FIG. 8 shows a full three-dimensional simulation of the total leakagecurrent of the top NMOS transistor 74 in FIG. 7, where a fin width of 15nm, a typical value for the 22 nm node, has been chosen.

The vertical axis 80 shows the leakage current I_(leak) in Amps/μm.

The horizontal axis 82 shows the supply voltage V_(dd), in Volts. Curve86 shows the simulated diode characteristic for the top NMOS transistor74 of FIG. 7, which has a 10 nm wide high-k dielectric layer of Si₃N₄(k=7.5). Curve 88 shows the simulated diode characteristic for an NMOStransistor constructed in the same way as the top NMOS transistor 74 ofFIG. 8 but with a very high-k dielectric liner (k=20). Curve 84 showsthe simulated diode characteristic for the reference case of an NMOStransistor without a liner along the vertical source/drain to substratediodes.

The leakage current I_(leak) is normalised to the effective width of theFinFET (W_(eff)=2H_(fin)+W_(fin)), where W_(eff) is the effective widthof the fin, H_(fin) is the height of the fin, and W_(fin) is the widthof the fin. The low-power leakage target is indicated for the 22 nmnode.

Again the graph demonstrates that the high-k liner reduces transistorleakage by about 1.5 order of magnitude, but what is even more importantis that the normalized (to the total effective width of the FinFET)leakage can be reduced to below 10 pA/μm, which, as shown in FIG. 8, isthe LP leakage target specification 90 for the low-standby power (LP)technology. In other words, the invention may facilitate the use of bulkFinFETs for low-power applications.

Although in the above embodiments the substrate 20 has been described assilicon, the substrate 20 can be any suitable substrate materialcompatible with integrated circuit fabrication processes on bulk planarwafers. Similarly, although the STI 18 has been described as comprisingSIO₂, the STI can be any suitable isolation material compatible withintegrated circuit fabrication processes on bulk planar wafers.

Although the fin 12 has been described as rectangular shaped, the finmay also have other shapes. For example the fin may have slightlyrounded top corners. The fin 12 can also be slightly tapered reducing inwidth towards the bottom. The fin 12 has been described as extending 20nm above the STI 18 and as being 20 nm wide. The fin can be of differentwidths and heights as desired for the particular application. A typicalrange for the fin width is 10-30 nm.

Although the figures show a tilted source-drain implementation in whichthe n/p junction 34 is located slightly below the STI 18 surface,FinFETs with n/p junctions located higher or lower in the fin 12 arealso envisaged.

The embodiments have been described with reference to NMOS devices.However, the invention is equally applicable to PMOS devices. Thusalthough the source and drain regions 14, 16 have been described asbeing heavily doped with n type dopants, they could also be doped withp-type dopants.

Whilst the high-k layer 26, 28 has been explained as being comprised ofHfO₂, (for which k=21), the high-k dielectric material may be anyinsulator compatible with CMOS fabrication technologies with a higherdielectric constant k than that of SiO₂. For example, another suitablematerial is Si₃N₄ (for which k=7.5). More generally, any material with ak value higher than that of SiO₂, i.e. k=3.9, may be used. For example,a k-value of k=5 may be used, to provide redistribution of the potentialdrop across the diode etc., to at least some extent, even though highervalues such as, for example, k=7.5 (for Si₃N₄) and k=21 for HfO₂, willtend to provide an even larger improvement. k=21 is the k-value of bulkHfO₂, although deposited layers may sometimes have values differingslightly therefrom. Generally, k-values of k≧20 are particularlyadvantageous. Examples of other possible materials for the high-kdielectric layer include HfSiO, ZrO₂, ZrSiO, and SrTiO₃.

Although the high-k dielectric layers 26, 28 have been described asbeing located on both side faces of the fin adjacent the junctions, itis also possible to place the high-k layer only on one side of the fin.

In the embodiment illustrated in FIGS. 1 and 2, the layer of high-kdielectric material lies beneath the full extent of the SiO₂ of the STItrench and completely surrounds the lower portion of the fin 12. It isalso envisaged, however, that the layer of high-k dielectric materialcould extend only partially under the SiO₂ of the STI trench.Alternatively, the layer of high-k dielectric could extend only alongpart of the length of the fin 12 adjacent the p/n junction 34.Furthermore, it is also envisaged that the layer of high-k dielectricmaterial could also extend above of the surface of the STI trench,partly to cover the upper portion of the fin.

Whilst the high-k layer 26, 28 has been described as being 5-10 nmthick, it is also possible for the layer to be thicker or thinneraccording to the desired application.

Although the leakage current has been described as being reduced by afactor of 30, according to the type and form of dielectric layer, theleakage current may be reduced by a factor of between 10 and 100.

Whilst the above embodiments are devices with 32 nm node CMOSfabrication, and the invention is particularly suited to 32 nm andbeyond technology, nevertheless the invention is also applicable toother node specifications, for example 22 nm node fabrication.

In terms of fabrication, although the step of providing the fin has beendescribed as by etching, any other suitable methods of providing thefin, such as by machining, could be used.

Additionally, the step of providing the high-k dielectric layer has beendescribed as by depositing, any suitable way of providing a layerbetween the fin and the STI on one or two opposite sides of the fincould be used.

Similarly the STI layer and the gate may be provided by any suitablemethod, such as deposition or coating. The high-k dielectric layer maybe planarised before the STI is planarised or at the same time. The STImay be etched back after planarisation before the high-k dielectriclayer is etched back so that the side walls of the fin are exposed or atthe same time.

The gate may be provided by any suitable method such as deposition andetching or stacking.

Whilst the step of doping has been explained as occurring after the gatestacking and etching, various doping stages may take place earlier. Forexample, the p-type ion implantation to form the punch through stopper(PTS) may take place before the deposition of the k-type dielectric.

Other doping steps, such as forming a well of p-type silicon under thePTS layer, may also take place before the gate etch.

Typical concentrations of dopants may be: source/drain 14, 16: 10²⁰atom/cm³; channel region 24: <10¹⁷ atom/cm³; punch-through stopper (notshown): 10¹⁸-10¹⁹ atom/cm³; well (not shown): 10¹⁶-10¹⁷ atom/cm³.

Other doping concentrations may also be selected according to thedesired application.

It should be noted that terminology such as top, over, above, under,below vertical, horizontal are used throughout the description, for thepurpose of explaining the relative positions of the features of thepresent invention. These terms are not intended to limit the orientationof the device.

The invention claimed is:
 1. A fin field effect transistor (FinFET)comprising: a fin having an upper portion and a lower portion extendingabove a substrate, the upper portion being doped with a dopant of afirst conductivity type, the lower portion being doped with a dopant ofa second conductivity type different from the first conductivity type,wherein a junction between the upper portion and the lower portion actsas a diode; at least one layer of high-k dielectric material adjacent toat least one side of the fin, wherein the high-k dielectric materialextends in a first plane parallel to the fin and does not extend in asecond plane in perpendicular to the fin, wherein the second plane is ata bottom plane of the fin; and a shallow trench isolation layer providedabove the substrate and adjacent to the layer of high-k dielectricmaterial, wherein the junction is between a top surface of the shallowtrench isolation layer and a bottom surface of the shallow trenchisolation layer.
 2. The FinFET according to claim 1, wherein the atleast one layer of high-k dielectric material has a k value of k≧5. 3.The FinFET according to claim 1, wherein the at least one layer ofhigh-k dielectric material has a k value of k≧20.
 4. The FinFETaccording to claim 3, wherein the at least one layer of high-kdielectric material is HfO₂.
 5. The FinFET according to claim 1, whereina top surface of the high-k dielectric material is level with a topsurface of the shallow trench isolation layer.
 6. The FinFET accordingto claim 1, wherein the high-k dielectric material does not extend to atop surface of the substrate.
 7. The FinFET according to claim 1,wherein the high-k dielectric material surrounds a portion of the lowerportion of the fin.
 8. The FinFET according to claim 1, wherein thehigh-k dielectric material surrounds the junction between the upperportion and the lower portion.
 9. The FinFET according to claim 1,wherein the fin further comprises a source and a drain separated by achannel region, the channel region of the fin being surrounded by a gateregion on three sides.
 10. The FinFET according to claim 1, wherein thelayer of high-k dielectric material extends from a position which isabove a bottom surface of the shallow trench isolation layer upwardly toa position which is level with a top surface of the shallow trenchisolation layer.
 11. The FinFET according to claim 1, wherein a portionof the high-k dielectric material covers the junction and the portion ofthe high-k dielectric material is embedded in the shallow trenchisolation layer.
 12. A fin field effect transistor (FinFET) comprising:a fin having an upper portion and a lower portion extending above asubstrate, the upper portion being doped with a dopant of a firstconductivity type, the lower portion being doped with a dopant of asecond conductivity type different from the first conductivity type, anda junction between the upper portion and the lower portion acts as adiode; at least one layer of high-k dielectric material adjacent to atleast one side of the fin, wherein the high-k dielectric materialextends in a first plane parallel to the fin and does not extend in asecond plane in perpendicular to the fin, wherein the second plane is ata bottom plane of the fin; a shallow trench isolation layer providedabove the substrate and adjacent to the layer of high-k dielectricmaterial, wherein a portion of the high-k dielectric material covers thejunction and the portion of the high-k dielectric material is embeddedin the shallow trench isolation layer; a gate region formed on top ofand around the sides of the fin; a channel region formed below the gateregion; and a punch through stopper layer provided in the lower portionof the fin below the channel region.
 13. The FinFET according to claim12, wherein the at least one layer of high-k dielectric material has a kvalue of k≧5.
 14. The FinFET according to claim 12, wherein a topsurface of the layer of high-k dielectric material is lower than a topsurface of the fin and higher than the junction.
 15. The FinFETaccording to claim 12, wherein the high-k dielectric material isembedded in the shallow trench isolation layer and the junction isbetween a top surface of the shallow trench isolation layer and a bottomsurface of the shallow trench isolation layer.
 16. A method offabricating a fin field effect transistor (FinFET), the methodcomprising the steps of: etching (s4) the substrate to provide a finextending above a substrate, wherein the fin has an upper portion and alower portion, the upper portion being doped with a dopant of a firstconductivity type, the lower portion being doped with a dopant of asecond conductivity type different from the first conductivity type,wherein a junction between the upper portion and the lower portion actsas a diode; depositing (s6) a layer of high-k dielectric materialadjacent to at least one side of the fin, wherein the high-k dielectricmaterial extends in a first plane parallel to the fin and does notextends in a second plane in perpendicular to the fin, wherein thesecond plane is at a bottom plane of the fin; depositing (s8) a shallowtrench isolation layer above the substrate and adjacent to the layer ofhigh-k dielectric material, wherein the junction is between a topsurface of the shallow trench isolation layer and a bottom surface ofthe shallow trench isolation layer; providing (s10) a gate region on topof and around the sides of the fin; and implanting dopants (s12) in thefin to form the active semiconductor areas.
 17. The method offabricating the FinFET according to claim 16, wherein the step ofdepositing (s6) a layer of high-k material comprises depositing (s6) atleast one layer of high-k dielectric material adjacent at least one sideof the fin for redistributing a potential drop more evenly over thediode, compared to if the at least one layer of high-k dielectricmaterial were not present, when the upper portion is connected to afirst potential and the lower portion is connected to a second potentialthereby providing the potential drop across the junction.
 18. The methodof fabricating the FinFET according to claim 16, wherein the step ofdepositing (s6) a layer of high-k dielectric material comprisesdepositing a layer of high-k dielectric material with a k value of k≧5.19. The method of fabricating the FinFET according to claim 16, whereinthe step of depositing (s6) a layer of high-k dielectric materialcomprises depositing a layer of high-k dielectric material with a kvalue of k≧7.5.
 20. The method of fabricating the FinFET according toclaim 16, wherein the step of depositing (s6) a layer of high-kdielectric material adjacent at least one side of the fin comprisesdepositing (s6) a layer of high-k dielectric material adjacent oppositesides of the fin.